The invention concerns a method for producing a highly-porous, ceramic layer which can be applied to metallic, ceramic, enameled and/or glass substrates using porous, ceramic particles, preferably aluminum oxide, titanium oxide and zirconium oxide and an inorganic binder system. The inorganic binder system contains at least one ceramic nanoparticle of a particle size of less than 100 nm, preferably less than 50 nm and particularly preferred less than 25 nm, the solvent being water. Layers produced in this fashion are suited for self-cleaning catalytically active layers e.g. in ovens, in combustion engines etc. or for general coating of substrates, to considerably increase their specific surface area e.g. for catalytic applications.
Ovens contain a cooking chamber which is lockable by a door and is delimited by an oven muffle. During roasting and baking, the side walls of the cooking chamber are soiled e.g. by splashing fat or meat juice or the like. This soiling during baking and roasting cannot be prevented. For this reason, the manufacturers have proposed several ways to clean the walls, top and bottom, i.e. the inner space of the cooking chamber. One generally differentiates between catalytic and pyrolytic cleaning.
For pyrolytic cleaning, the cooking chamber has so-called grill rods which can be controlled and heated through a separate, electronically controlled program and are preferably mounted to the top of the cooking chamber. The organic soiling is carbonized, i.e. completely burnt, at temperatures of more than 500° C. (Cepem Cie Euro Equip Menager [FR2605391] or Bosch Siemens Haushaltsgeräte GmbH [DE2526096]. Pyrolytic cleaning is demanding and expensive due to the required high temperatures. Ovens with pyrolytic cleaning must have suitable protective mechanism to block the door of the cooking chamber during pyrolysis (from approximately 320° C., Bosch Siemens Hausgeräte GmbH [EP 0940631] to protect the oven from improper operation. Since these ovens furthermore require more expensive heating elements to be able to control the high temperature at all, pyrolysis systems have been established only in ovens of the top price bracket.
In view of the costs, the catalyst systems are preferable to pyrolysis systems since catalytic combustion of soiling takes place at lower temperatures, i.e. below 500° C. Matsushita Elec. Ind. Co. Ltd. [JP03056144] proposes lining of the interior of the oven with a catalytically active coating which consists of a binder system and a catalytically active powder. Metal oxides are used as catalyst, preferably manganese dioxide and silicon resins are used as binders. This catalytic coating permits cleaning of the oven interior already between 380° C. and 400° C. as stated by the manufacturer. Mixture of a catalyst and a binder system or a layer matrix for coating the inner surface of an oven can also be found with other oven manufacturers. Toshiba [JP60147478] uses manganese oxide or ferrite as catalyst and sodium silicate as binder phase. In an analog fashion, Sharp KK [JP54135076] uses quartz sand or sodium silicate as binder phase and iron oxide or copper oxide as catalyst. These protective rights give no statement about the effectivity of the two latter catalytic coatings. The onset temperature of the coating, i.e. the temperature at which the layer starts to work, was reduced in accordance with the above-mentioned documents to 270° C. to 300° C. (Toshiba) and even to 250° C. (Sharp KK). In practice this means, that there are catalytic coatings which start to break down fat etc. in the interior of the oven at temperatures below 320° C., however, the efficiency of the coating is not sufficient to completely finish this decomposition. After each baking or roasting cycle, residues of non-decomposed fat remain in or on the layer lining the interior of the oven, such that after a very short time, the function of the layer is impaired since it is varnished. For complete decomposition, these systems still require temperatures of usually more than 380° C.
Finally, NGK Insulators Ltd [JP56095022] should be mentioned, which use manganese oxide, copper oxide and iron oxide as catalysts, and a porous enamel as layer matrix to increase the amount of applied catalyst, as well as the protective rights of Matsushita [JP02069574], Cie Euripeenne pour L'Equ [FR2040822] and Hoover LtD [GB1177434] which all use flouropolymers as carrier layer for the catalysts to minimize the surface energy of the carrier layer and prevent adhesion.
Pyrolytic cleaning is very effective at temperatures above 500° C. but is expensive due to the facts given by process technology. These systems are currently used only for ovens of the top price bracket (maximally 10% of all ovens). The reduction in cost promoted the development of catalytic cleaning. The inner walls of the cooking chamber are thereby lined with a layer which always contains a catalyst. Suitable catalysts are manganese oxide, iron oxide and copper oxide, wherein temperature-resistant polymers, sodium silicate, quartz sand and enamel are used as binder phase of the catalyst or as layer component. The catalysts operate at temperatures of more than 380° C. which requires safety measures producing additional costs. Only a few catalytically active coatings are known whose onset temperature, i.e. start of fat disintegration in the layer is between 250° C. and 350° C. In these cases, large amounts of residues remain in or on the layer during permanent operation of the oven below 350° C. with the consequence that these oven inner coatings varnish very quickly.
Catalysis is subject to thermo-dynamic rules. A catalyst cannot change the thermodynamics of a system but only lower the activation energy, i.e. the tendency to start the reaction. Although combustion of the organic soiling occurs thermodynamically only at a higher temperature, it starts at a lower temperature if initiated by a catalyst. Not all parts of the organic soiling disintegrate at this low temperature, which leaves residues which cause varnishing of the interior of the oven with the consequence that the optic and haptic appearance of the oven interior drastically deteriorates after only a few baking and roasting cycles.
It is the underlying purpose of the invention to develop a coating for the interior of an oven which automatically eliminates the soiling produced through roasting and baking, i.e. through application of a temperature of considerably less than 320° C., wherein the working temperature of the layer is preferably 250° C.